1. Field of the Invention
The present invention relates to optical network terminals and, more particularly, to an optical network terminal with illegal transmission detection circuitry.
2. Description of the Related Art
A fiber-to-the-x (FTTx) passive optical network (PON) is a point-to-multipoint communications network. Examples of an FTTx network include a fiber-to-the curb (FTTC) network and a fiber-to-the-home (FTTH) network. In an FTTx network, downstream data packets are transmitted from an optical line terminal (OLT) to a number of optical network terminals (ONT) that are located at or near a corresponding number of end users. Upstream data packets, on the other hand, are transmitted from the ONTs back to the OLT.
FIG. 1 shows a block diagram that illustrates a prior-art example of an optical network terminal (ONT) 100. As shown in FIG. 1, ONT 100 includes an optical transmitter 110 that generates an upstream data packet DP in response to an outgoing data packet DTX, and determines an output power level in response to a measured power signal TPC. In addition, optical transmitter 110 outputs the upstream data packet DP with the output power level when a transmit enable signal EN is asserted.
As further shown in FIG. 1, ONT 100 also includes a photo detector 112 that detects when optical transmitter 110 is transmitting. Photo detector 112 measures the transmit power level output by optical transmitter 110, and outputs the measured power signal TPC to optical transmitter 110 to indicate the measured transmit power level. Transmitter 110 can then modify the transmit power level as needed to insure that the correct transmit power is output.
ONT 100 additionally includes a power supply 114 that provides the necessary voltages and currents to optical transmitter 110, and a media access controller (MAC) 116 that generates the outgoing data packet DTX in response to outgoing subscriber data SD, asserts the transmit enable signal EN when transmission is enabled, and de-asserts the transmit enable signal EN when transmission is disabled.
One problem which can arise with an ONT, such as ONT 100, is when an ONT fails due to hardware or software problems which cause the ONT to transmit at the wrong time. If a first ONT transmits during a time slot which has been assigned to a second ONT and both ONTs attempt to transmit at the same time, a collision results which can cause the data packets output by both ONTs to be lost.
Thus, when the optical transmitter of an ONT “sticks on” and continuously transmits during all of the assigned time slots, the entire FTTx network can be rendered incapable of any communication between the ONTs and the OLT. An ONT that transmits in the upstream direction during the wrong time slot, such as a continuously transmitting ONT, is known as a “rogue ONT”. Thus, in order to prevent a rogue ONT from incapacitating an entire a FTTx network segment, there exists a need for an ONT that can detect when it is illegally transmitting, and turn itself off.